SMC (sheet molding compound) used to make FRP parts usually contains about 30% by weight of glass fibers, a large amount of filler(s) and a low shrink additive(s). HMC (high strength molding compound) used to make FRP parts (which have an Izod impact resistance about twice that of FRP parts made from SMC) usually contains about 55 to 60% by weight of glass fibers but does not contain any, or any appreciable amount of, fillers and low shrink additives. Because of the high glass content and lack of fillers and low shrink additives, the HMC part in the mold during curing (at about 300.degree. F. or 149.degree. C. and 600 p.s.i.) is under considerable stress, as compared to the SMC part (molded and cured under the same conditions), due to polymerization shrinkage and the usual inhomogeneous distribution of glass fibers throughout the polyester resin composition. Moreover, on removal from the mold the cured HMC part thermally shrinks more than the cured SMC part at room temperature (about 25.degree. C.), and is more distorted and has a poorer surface smoothness than the cured SMC part. For example, the molded and cured SMC part will generally exhibit a profilometer reading (surface roughness--distance between peaks and valleys) of about an average of 100 microinches while the molded and cured HMC part will exhibit a profilometer reading of an average of about 750+ microinches.
Thus, when in-mold coating a HMC molded and cured part according to the high temperature and high pressure in-mold coating process of U.S. Pat. No. 4,081,578, even through the coating may conform perfectly to the cavity of the mold during molding, the HMC part and its in-mold coating will buckle and deform after the mold is opened and the part cooled.
It, therefore, is an object of the present invention to attempt to overcome the difficulties alluded to hereinabove and to provide a method for in-mold coating a thermoset glass fiber reinforced polyester resin part (FRP) such as a HMC part which exhibits an improved surface.
These and other objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description and working example.